Radioactive source comprising a sheet article containing a layer of small discrete radioactive beads



April 2, 1968 A58 3,376,422

INING D. L. H RADIOACTIVE SOURCE COMPRISING A SHEET ARTICLE CONTA A LAYER OF SMALL DISCRETE RADIOACTIVE BEADS Filed July 15, 1964 I NVENTOR 006 1410 A flfiES :1 wf/ghnu 4770 N575 United States Patent 3,376,422 RADIOACTIVE SOURCE COMPRISING A SHEET ARTICLE CONTAINING A LAYER OF SMALL DISCRETE RADIOACTIVE BEADS Donald L. Haes, Mounds View, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed July 15, 1964, Ser. No. 382,842 8 Claims. (Cl. 250-106) This invention relates to new articles useful as area sources of radioactivity and to a method of making the same. More particularly, the invention is directed to sheet articles of radioactive character.

Radioactive sheet articles of course are not new. Heretofore, however, the structures employed for sheet sources of radioactivity have been relatively susceptible to loss of radioactive substance from the sheet structure to the environment after a varying but relatively short period of time. Radioactive substance has been only poorly entrapped in such structures; and degradation of portions of the structure is followed by rather rapid loss of radioactive substance. Danger of contamination exists not only during the last portion of the useful life of such structures, but also during the time such structures are collected and transported for disposal in burial grounds.

Radioactive sheet articles of the present invention contain a layer of small discrete radioactive source articles in the shape of hard and brittle beads or spheroids. The concepts of the invention permit the formation of radioactive sheet structures allowing for very high escape of radiation (e.g., alpha particles) therefrom, while at the same time limiting or inhibiting migration of radioactive material therefrom. Thus, the sheet articles hereof are relatively safe for necessary handling during their relatively long use life as well as during collection and transport to burial grounds. The spheroidal radioactive particles used in forming the sheets are themselves handleable without serious dust or air-borne contamination problems. They become an integral part of the sheet articles of the invention, and are inseparable therefrom even after extended use-exposure of the material of the sheet articles to radiation.

Particularly desirable spheroids for use in practicing the invention are those formed by dispersing a radioactive isotope by ion exchange within pores of an inorganic matrix having a bead shape, and then fixing the radioactive material within the pores by a heating step eifective to cause shrinkage of the pores such that the radioactive isotope in dispersed ionic state is mechanically lodged and trapped within the matrix. Radioactive inorganic bead articles of this type are taught and claimed in copending U.S. Ser. No. 712,254, filed Jan. 30, 1958 by John P. Ryan, now U.S. Patent No. 3,147,225. Their matrix is characteristically at least partially crystalline in nature (e.g., glassy matrices are at least partially 'devitrified). The most preferred inorganic composition for the matrix includes an oxide of one or more of titanium, zirconium and hafnium, plus an oxide of phosphorus, with complexes such as titanium phosphate, zirconium phosphate and hafnium phosphate being presumptively present in the complex inorganic structure of these matrices. Such refractory beads containing radioactive material are preferably so refractory that they do not sinter or stick to one another when a mass of the same is subjected to 1000 C. with beadsin contact with one another. Further, the resultant radioactive-containing refractory beads are such that they usually can withstand water exposure for one week at 50 C. with less than 0.1% weight loss of the radioactive, isotope entrapped within shrunken pores thereof.

Quite satisfactory hard and brittle radioactive beads or spheroids for practice of the invention may also be formed from porous beads of ion exchangeable organic material (e.g., Dowex-SO, a sulfonated polystyrene-divinyl benzene). Radioactive material is placed within the pores of such beads by ion exchange; and this is followed by heating the resulting radioactive-containing beads to about 450 C. to cause shrinkage of the pores thereof and conversion of the ion exchangeable beads into hard and brittle essentially-non-ion-exchangeable carbonaceous (but still partially organic) structures of bead form. These carbonaceous beads are relatively stable at elevated temperatures up to about 450 C. or even somewhat higher; they are non-melting and remain as discrete entities with radioactive material trapped within the shrunken pores thereof.

If desired, practice of the invention may be accomplished using hard and brittle beads formed by still other means, whether the beads are dense or not so dense in character, and whether they are crystalline or noncrystalline or amorphous in structure, provided they have the required radioactive isotope material entrapped therewithin.

Sheet articles of this invention have spheroidal elements as aforenoted pressed into a metal substrate layer to a partial extent, at least an amount equal to half their diameter up to about or even 98% of their diameter. The metal of the metal substrate layer is forced upwardly between and about the spheroidal elements during pressing and grips the elements and holds them against dislodgment. No other bonding means to hold the spheroids in place is needed. Optionally, sheet articles of this invention may contain an overcoating of solids organic plastic material, and may also be provided with a metal screen over the spheroid-containing surface (and spaced from the material of that surface) such that contact with the spheroidal elements and organic plastic of the strucure is largely prevented. A metal screen is especially desirable on alpha sheet sources inasmuch as alpha emitting radioisotopes (polonium 210) generally also exhibit a tendency to migrate (especially as compared to the relative low tendency toward migration exhibited by beta and gamma emitting radioisotopes); and a screen serves as an external guard against contact with the sheet surface of the alpha source. But a screen or other additional elements of structure are not needed to hold the spheroids in place. The spheroids themselves are gripped and held in place by the metal into which they are pressed.

Alpha emitting sheet articles of the invention are especially useful in static eliminator applications. Voltage charges about traveling webs may be reduced essentially to zero by air ionization as eifected by using alpha emitting sheet articles of the invention in static eliminator applications.

The sheet articles are also useful to create sheet sources of radioactivity (e.g., alpha, beta, and gamma radiations) useful in any environment where controlled radioactive emissions are desired. Sheet articles may be formed into a source capable of emitting beta rays with little or no bremsstrahl-ung radiation. For example, two sheet articles according to the invention, with Strontium-9O fixed in the beads or spheroids thereof, when placed with their bead-containing faces in juxtaposition and the peripheral areas of the sheets welded or soldered together, serve to create a source which emits a significantly higher beta radiation than conventionally obtained with an equivalent quantity of Strontium90; and this is accomplished with a significant drop of bremsstrahlung radiation.

By coating phosphor over the bead-containing surface of sheet articles hereof, suitably in a binder composed of organic plastic solids, self-luminous structures may be formed. Indeed, sheet articles of the invention may even be useful as temperature detectors where the detection is accomplished by noting the release of radioactive-containing beads from a melted substrate metal whose temperature of melting has been predetermined.

The problem of making radioactive sheet articles according to the invention is by no means one to which a solution is presented by the prior art. Merely pressing radioactive-containing beads into a metal substrate layer might at first blush appear to be the solution; but the results obtained, particularly when refractory beads containing radioactive material within shrunken pores are used, are not always desired inasmuch as the beads tend to be crushed if not pressed into the substrate under special temperature conditions, or tend to be crushed by rubbing a steel pointer there-over even after being placed in the substrate under special temperature conditions.

Thus, in essential respects, the more preferred refractory inorganic beads for use in practicing the invention might be looked upon as somewhat fragile or friable under severe compressive forces, although they are not readily friable under most, if not all, conditions of use to which they have heretofore been subjected. It appears that the stresses acting upon the beads after being placed in the metal substrate layer are such that sometimes only a light further force becomes necessary to effect breakage of the same. According to the invention, the deficiency just noted is entirely obviated by employing a special plating about the bead elements.

Further details of the invention will now be described with particular reference to a drawing made a part hereof wherein:

FIGURE 1 is a perspective schematic view of the significant features of a static eliminator formed according to the invention;

FIGURE 2 is a cross-sectional schematic view illustrating a sheet of the invention, useful as part of the static eliminator of FIGURE 1; and

FIGURE 3 is a cross-sectional schematic view of a modified sheet structure of the invention.

Referring first to FIGURE 2, the basic sheet article hereof comprises a malleable and ductile metal substrate layer having a Brinell hardness number no greater than at 300 C. and no greater than 75 at room temperature, plus a layer, preferably a compact monolayer, of a multitude of discrete spheroidal elements between 5 and 250 microns in diameter pressed partially int-o the metal substrate layer to such an extent that at least half the diameter of the elements is below the surface of the metal substrate layer. Preferably the elements are pressed into the metal substrate in excess of one-half their diameter (e.g., or of their diameter). These spheroidal elements consist essentially of a bead or spheroidal brittle core member 11 containing radioactive material fixed therein, and a concentric metal veneer plating 12 thereabout. The plating is between about 0.2 and 5 microns thick, preferably between about 0.4 and 2 microns thick, and never greater in thickness than approximately 10% of the diameter of the spheroidal core. It is preferably so thin that it provides little noteworthy interference to penetration by radiation from entrapped radioisotopes within the core.

A significant relationship exists between the metal of the veneer plating about the cores and the metal of the metal substrate layer into which the plated cores are pressed. The metal of the veneer plating is less ductile than the metal of the metal substrate layer. It has a Brinell hardness number in excess of the hardness number of the metal of the metal substrate layer.. Usually the Brinell hardness number for the metal of the veneer plating will be in excess of 100 at room temperature and in excess of about 75, or even 100, at 300 C.; but the criti- Cal relationship is that the Brinell. hardness number for the veneer metal is in excess of that for the substrate metal layer.

Of significance also is the fact that the ductility of the metal of the veneer plating about the cores is greater than the ductility of the brittle material of the spheroidal cores which contain the radioactive material. Further, the Brinell hardness number for the metal of the veneer plating is frequently (but not always) greater than that for the brittle spheroidal cores. (The material of the spheroidal cores, if in dense form, might well be harder than the metal plating; but in preferred inorganic structures, the cores are collapsed or shrunken matrices which, as such, exhibit lower Brinell hardness numbers than the metal plating.) These noted preferred relationships between the metal of the veneer plate and the metal of the substrate layer and the material of the core are true at both room temperature and at elevated temperatures such as about 300 C. (or, in general, any other elevated temperature up to about 500 C.to which the components of the sheet article are subjected during manufacture and possible use).

In the most desirable structures according to the invention, the content of radioactive material fixed in the spheroidal cores, and the concentration of the cores per square inch of the metal substrate layer, are such that at least 0.5 millicurie of radioactive material per square inch of surface of the structure is present. Suitable structures, however, may be formed with as little as, but at least as much as, one microcurie of radioisotope per square inch of the sheet structure. Such lower levels of radioactivity may be used in sheets for demonstration purposes, particularly educational purposes.

Quite significant is the fact that the spheroidal elements (core with metal plating) are each gripped laterally in the layer structure and are held in the metal substrate layer by laterally compressive forces exerted thereupon by the metal of that layer. Thus it is that the metal plating layer performs the desirable function of protecting the core against direct compressive forces, and the desirable function of serving as a reinforcement to the core in connection with the laterally compressive forces transmitted through the plating to the core.

Over the sheet structure may be applied an additional layer 13, as particularly illustrated in FIGURE 3. Organic plastic material may be used to form this additional layer. For example, a coating of any of a variety of thermoplastic (e.g., vinyls, acrylics, etc.) or thermosetting (epoxies, phenol-aldehydes, etc.) organic materials rnay be used. Solid organic materials present relatively little hindrance to the passage of radioactive emission therethrough- If desired, inorganic coatings such as metals, glazes, or other inorganic materials (sodium silicate) may be applied as an additional layer to the sheet structure. Further, if desired, a layer of a suitable low atomic number material (e.g., beryllium) may be placed in juxtaposition (e.g., as an additional layer) with an alpha emit-ting sheet source of the invent-ion to form a neutron generating surface or source.

Where desired for sign purposes or self-illumination, a phospho activated by the particular radioactive eman-ations from the structure may be applied as a discontinous or continuous layer 14' (see FIGURE 3), either preliminary to, or with, the application of an additional overcoating layer. For example, a zinc sulfide phosphor, which is excited by beta particles such as those from Pm147, may be applied over a sheet article containing Pm147 entrapped in spheroids.

As illustrated in FIGURE 1, sheet sources of the invention (whether coated with organic solids or not) may be covered over their bead-containing surface with a screen material 15 (shown as partially broken away) spaced from the materials of the sheet and affixed about the periphery of the sheet. Such a structure gives desirable results according to the wipe test specified by the Atomic Energy Commission (involving wiping a moistened filter paper on the most accessible surface, that is the screen, of a composite radioactive source, and then placing the paper used .in wiping in an appropriate counter and calculating the number of microcuries of radioactive material removed per wipe, it any).- After a period of time some radioactive migration may occur in almost all structures, and such migration has been recognized as quite serious with respect to Polonium-ZIO; thus the structure illustrated in FIGURE 1 may readily be recognized as one allowing for a relatively long useful life as determined using the standards of the well known wipe test. In basic respects, however, a static eliminator having a structure as illustrated in FIGURE 1 comprises a strip of radioactive sheet material 16 as described herein, with alpha-emitting radioactive material as a part thereof, and suitable frame means 17 to hold the strip 16 of radioactive source material within the recess of a bar member 18. Over the bead face of the strip of radioactive source material may be placed a metal screen with advantages as aforenoted.

It should be noted that the essential features of each of the elements or parts of the articles hereof may be satisfied using a mixture of ingredients. Thus, the metal of the metal substrate layer as well as the veneer plating may be alloys. A veneer plating of more than one metal may be employed in stratified layers. The metal substrate layer may be very thin (at least as thick as approximately half the diameter. of the discrete spheroidal inorganic elements pressed partially the-rein) and affixed as a coating or veneer upon a base material having properties the same or different from those required for the metal substrate layer. For example, a thin coating of aluminum having a thickness equal to approximately half of the diameter of inorganic spheroids pressed therein may be supported or afiixed to an underlying steel substrate. Also, more than one radioactive isotope may be placed in a spheroidal core, or mixtures of spheroidal cores containing different radioactive isotopes may be employed.

In making the sheet articles hereof, a metal substrate layer having the properties as aforenoted is first selected. Usually this substrate layer or sheet will not exceed a thickness of about 4 inch, or even not exceed about 50 mils in thickness. Generally it will be at least about three or four mils thick. Upon this substrate layer is formed a monolayer of discrete spheroidal elements comprising a core with radioactivity fixed therein and -a metal veneer plating as aforedescribed thereabout. The size of the spheroidal elements (.i.e., plated beads) should range from about 5 microns up to about 250 microns in diameter. It'is emphasized that they are spheroidal in nature and not irregular angular particles as obtained by grinding. Further,'the mean size of. 90 weight percent of the spheroidal elements should fall Within a limited size range with the largest being no larger in diameter than approximately 30% greater than the diameter of the smaller particles within that mass of 90 weight percent of the elements. This preferred size. distribution is especially desired Where the spheroidal elements are to be pressed into the metal substrate layer to a distance equal to approximately half their diameter o only slightly more. Asa criteria, it becomes less significant as the elements are pressed into the 'metal'substrate layer up to 80% of the diameter of the larger elements or even more, say 90 to 98% of the diameter; If a mixture of spheroidal elements varying greatly in size is employed, some become substantially embedded (although possibly not completely embedded) within the metal substrate layer and therefore are rendered less eifective from the standpoint of their emissions escaping from the structure. Thus, it is much preferred to employ elements within the criteria of limited size range as aforenoted so as to gain resulting structures exhibiting maximum efiiciency of emission.

Preferably, also, the monolayer of 'discrete spheroida inorganic elements is applied to the metal substrate layer after first applying thereover an intermediate coating having sufiicient tackiness to hold the applied layer of discrete spheroidal elements against dislodgement during handling of the structure preliminary to pressing the spheroidal elements into the substrate layer. A suitable tacky layer is formed by coating the metal substrate layer with a thin film of oil, or other normally tacky material, or a solvent-tackified material.

Then the monolayer of elements is pressed into the metal substrate under elevated temperature conditions which are insulficient to cause melt flow of the metal substrate layer, and insufficient to cause dissipation of the radioactive material employed. By elevating the temperature of the metal substrate layer to some-extent during the pressing step, the conditions of pressure needed for pressing the spheroidal elements into the substrate are desirably reduced. Generally, however, even at elevated temperatures approaching the melt-flow temperature of the metal substrate layer, at least approximately 2000 p.s.i. is needed to press the spheroidal elements effectively into the metal substrate to a distance at least equal to about half their diameter. Pressures. in excess of about 40,000 p.s.i are undesirable, as the danger of cracking the cores containing the radioactive material increases as pressure increases; and this of course means that the danger of creating an unwanted hazard of radioactive contamination at a later date is increased. Indeed, the pressures desired are not in excess of 20,000 psi, and preferably not in excess of 10,000 psi. Very desirable results have been obtained in the case of aluminum substrates by employing pressures of approximately 5,000 p.s.i. and temperatures of about 350 C.

Once the pressure is removed and the metal substrate layer is allowed to cool, compressive forces are exerted upon the spheroidal elements pressed into the substrate layer. Significantly, the elements pressed at least approximately half their diameter into the layer are effectively gripped by the layer inasmuch as the lateral compressive forces exerted by the layer seem to predominate.

A further step, as a matter of course, is that of subjecting the surface of the resulting structure which contains the inorganic elements pressed therein to cleaning action so as to remove any incidental removable radioactive material therefrom. If desired, an additional coating (such as methyl methacrylate) may be applied, usually at a thickness not in excess of about 50 mils, preferably not in excess of about 1 to 2 mils.

A preferred sheet structure of the invention may be made as follows:

First, the spheroidal elements containing radioactive material are formed. In accomplishing this a raw material batch of inorganic oxides having a compositional analysis in mol percent consisting of about 6 .0% ZrO A10 33.0% B0 20.0% P0 and 32.0% NaO is melted to a homogeneous mass at about 1350 C., and quenched in a cool bath of water. Resulting glass particles are dropped through a radiant heating zone where they soften sufficiently to permit surface tension forces to form them into spheres while free-falling.

About .10 parts by weight of the glass beads are immersed in about 150 parts by weight of a five normal solution of nitric acid for about 6 hours, with continuous agitation at room temperature, after which the beads are washed with water. Resulting leached beads, on chemical analysis, show an inorganic composition, set forth in mol percent, consisting of about 33% Z rO about 2% A10 about 64% P0 and traces or small amounts of B0 3 and NaO Wash water is drained from the heads, but the beads are not dried. They have an ion exchange capacity of about 6.8 milliequivalents per gram, and an effective surface area of about 670 square meters per gram, as may be determined by the Brunauer-Emmett- Teller (B.E.T.) method.

About three grams of the leached beads, from which excess water had been drained, are then shaken for 24 hours with five millicuries of prometheum-l47 in the form of PmCl dissolved in ml. of a very dilute acid solution (about 0.1 to 1.0 normal HCl water solution). At the end of 24 hours, the supernatant isde- 7 canted and assayed to determine the percent of PM-147 adsorbed by the beads. About 99% of the PM-147 is found to have been absorbed by the beads.

The beads are then rinsed with water, dried in air a few hours, and raised to 500 C. over a period of about 4 hours. They are fired at 500 C. for about 18 hours, and then raised over a period of 3 hours to 1000" C. where they are maintained for about 4 hours, after which they are cooled to room temperature gradually over a period of about 16 hours. During firing, the leached pores throughout the beads contract and the ionically-bonded radioactive PM147 ions become an integral part of the structure. They are mechanically and chemically entrapped therein. X-ray diffraction analysis of the inorganic matrix of the bead indicates that a large percentage of the matrix is converted by the heating step from amorphous to a crystalline phase.

Then the beads are shaken for one hour with a 100 ml. aliquotof one normal aqueous H 80 to remove any small amount of PM-147 which may have clung to the outer surfaces of the beads. This acid wash is repeated as an added precaution to remove residual surface radioactive ions, and then the product is rinsed with water and dried in air.

Resulting beads are classified according to size and those within the size range of about 40 to 50 microns are selected for further processing. They may be surface activated by placing them in a stannous chloride solution (70 grams per liter concentration in water) for 10 mmutes, which results in the deposition of a thin layer (atomic thickness) of tin on the surface of the beads. Then they are placed in a silver nitrate solution (10 grams per liter concentration in water) for 5 minutes, which results in a thin deposit (atomic thickness) of silver on top of the tin layer. Thereafter they are placed in a palladium chloride solution (0.1 gram per liter concentration in water) for 5 minutes which results in the replacement of the silver deposit by a deposit of atomic thickness of palladium. Washing of the resulting beads in Water is then accomplished, followed by placing them in a nickel plating solution for about five minutes. A suitable aqueous nickel plating solution is one consisting of 30 grams per liter nickel chloride, 50 grams per liter sodium hydroxyacetate (a buffering agent), and grams per liter sodium hypophosphite (a reducing agent to reduce nickel ions to metallic nickel). Nickel plating solutions of this type are available commercially (e.g., Enplate 410). Initiation of the plating occurs catalytically by the action of the palladium deposited upon the spheres. About five minutes of exposure results in a nickel plating approximately 0.5 micron thick about the spheroidal cores. These nickel plated spheroidal elements are then sprinkled over a central strip area (/2 by 6 inches) of a 50 mil thick malleable and ductile aluminum sheet (about by 6% inches) consisting essentially of about 99% pure aluminum, with up to about 1% impurities. The peripheral /3 inch width areas of the sheet are masked off using masking tape; and the central strip area of the sheet, previously coated with a suitable sticky adhesive (e.g., an epoxy resin, an acrylic monomer, etc.) at about milligrams per square inch, is covered with a compact monolayer of the plated beads (such that the resulting source will have at least about 50 up to about 100 microcuries of radioactive matter per square inch). Those elements in excess of a monolayer are removed by brushing. This structure is then placed between the steel ram and backing plate of a vertical press preheated to about 350 C. (Prior to doing so, if desired, the rear surface of the substrate sheet may be coated with a release agent, e.g., a silicone release agent.) Then the spheroidal elements, while the assembly is at about 300-350 C., are subjected to a pressure of approximately 5,000 p.s.i. for about 2 minutes by means of the steel ram. This results in the spheroidal elements being pressed at about 60-70% of their diameter at the aluminum sheet.

Then the pressure is released, and the source removed from the press and allowed to cool. The bead-containing surface of the structure is then cleaned to remove any incidental loose radioactive material. Suitably, sponge application of a well-known cleaning agent (such as a mix ture of versene, water and fine alumina powder) is used for this purpose. Further cleaning is accomplished by washing with water and rinsing with acetone.

Next, if desired, a stainless steel screen, suitably of about 20 mesh to about 400 mesh and suitably provided with a central area raised relative to its peripheral rim areas, may be placed over the active area of the sheet article. If desired, the screen may be pressed firmly about its periphery into the peripheral areas of the metal substrate layer not containing partially embedded spheroidal elements; and the conditions of temperature and pressure to accomplish this are the same as those used for pressing the spheroidal elements into the substrate. The. time of pressing, however, need be only approximately one minute.

If a static eliminator is to be formed, the specific conditions of fabrication outlined above are duplicated except that Polonium-210, or other alpha emitter, is employed as the radioisotope, at a preferred concentration of about 0.5 to 5 millicuries per square inch.

Some illustrative metals useful as the metal substrate layer of sheet articles according to the invention, as well as their Brinell hardness numbers at room temperature and at certain elevated temperatures are set forth in Table I.

TABLE I Brinell Hardness Brinell Hardness Melting Metal N0. at Room No. at Specified Point Temperature Temperature 0.)

As the metal for the metal plating about radioactivecontaining cores, nickel is very much preferred. However, other metals satisfying the criteria aforenoted for this plating are equally suitable to employ. An illustrative further metal is cobalt which has a room temperature Brinell hardness number of about 125 and a Brinell hardness number of about 84 at 500 C. Cobalt melts at about 1495 C. Nickel, which melts at about 1453 C., exhibits a Brinell hardness number of 240 at room temperature and 180 at 500 C. It is to be recognized, of course, that the hardness of specific metal samples will vary somewhat depending upon the history of the samples.

Radioactive isotopes useful in forming spheroids for practice of this invention may vary essentially as desired. Where a source having a long life of beta emissions is desired, Sr-90 is especially useful inasmuch as its half life is 28 years, a considerable increase over the half life of Prn-147 (a beta emitter) which is about 2.6 years. Where an even shorter life is desired, Y-90 is desirably used as a beta emitter. A short lived alpha emitter is Po-210, having a half life of about 138 days. An alpha emitter having a long half life of about 458 years is Am-241. A most desirable isotope for gamma emitting sources is Cs-137 which has a half life of about years and causes the formation of Barium-137M, which emits gamma radiation but has a half life of only about 17 minutes.

That which is claimed is:

1. An article, useful as an area source of radioactivity, handleable without dangerous radioactive material transfer to ones fingers, said article comprising a malleable and ductile metal substrate layer having a Brinell hardness number no greater than at room temperature, and a compact monolayer of a multitude of discrete spheroidal elements between 5 and 250 microns in diameter pressed into said metal substrate layer to a partial extent at least equal to half the diameter of said elements, at least 90 weight percent of said elements being within a limited size range having an upper limit of diameter no larger than 30% greater than the lower size limit of diameter within said 90 weight percent portion of said elements, said spheroidal elements each consistingessentially of a hard and brittle spheroidal core containing radioactive material fixed therein and a reinforcing concentric metal veneer plating thereabout between 0.2 and microns thick and no thicker than about of the diameter of the core, said metal of said veneer plating being less ductile than the metal of said metal substrate layer and more ductile than the material of said brittle spheroidal core, said metal veneer plating further having a Brinell hardness number at room temperature in excess of that for the metal of said metal substrate layer, said spheroidal elements being each gripped laterally in said layer structure and held in said metal substrate layer by lateral compressive forces exerted thereupon by said metal of said metal substrate layer, the radioactive material fixed in said cores of said spheroidal elements and the concentration of said spheroidal elements per square inch of said metal substrate layer being such that at least one microcurie of radioisotope per square inch of surface of said sheet article is present.

2. The article of claim 1 wherein the metal substrate layer is aluminum.

3. The article of claim 1 wherein the metal veneer plating is nickel.

4. The article of claim 1 wherein the radioactive material fixed within the spheroidal core is Pm-147.

5. The article of claim 1 wherein the radioactive material fixed within the spheroidal core is Po210.

6. The article of claim 1 having a coating of organic plastic solids over the surface containing the spheroidal inorganic elements.

7. The article of claim 1 having a metal screen spaced from and affixed over the surface containing the spheroidal inorganic elements.

8. The method of making a sheet article useful as an area source of radioactivity and handleable Without dangerous radioactive material transfer to ones fingers, comprising (1) forming a monolayer of discrete spheroidal elements between 5 and 250 microns in diameter on a malleable and ductile metal substrate layer having a Brine'll hardness number no greater than at room temperature, at least of the weight of said elements being within a limited size range having an upper limit of diameter no larger than 30% greater than the lower size limit of diameter within said 90 weight percent portion of said elements, said spheroidal elements each consisting essentially of a hard and brittle spheroidal core containing radioactive material fixed therein and a reinforcing coucentric metal veneer plating thereabout between 0.5 and 5 microns thick and no thicker than about 10% of the diameter of the core, said metal of said veneer plating being less ductile than the metal of said :metal substrate layer and more ductile than the material of said brittle spheroidal core, said metal veneer plating further having a Brinell hardness number in excess of that for the metal of said metal substrate layer, the radioactive material fixed in said cores of said spheroidal elements and the concentration of said spheroidal elements per square inch of said monolayer being such that at least one microcurie of radioisotope per square inch of area of said monolayer is present; (2) pressing said compact monolayer of spheroidal elements into said metal substrate layer to a partial extent at least equal to half the diameter of said elements using conditions of pressure between 2000 p.s.i. and 40,000 p.s.i. and conditions of elevated temperature for said metal substrate layer up to, but not including, the temperature causing melt flow of the meta-l substrate layer such that said partially embedded spheroidal elements are gripped laterally by the metal of said metal substrate layer and are held in said metal substrate layer by said lateral gripping forces; (3) removing the pressure forcing said elements into said metal substrate layer and cooling said resulting structure to room temperature; and (4) cleaning the surface of said structure which contains the spheroidal elements pressed therein to remove any inci dental removable radioactive material therefrom.

References Cited UNITED STATES PATENTS 2,837,659 6/1958 Handee et al 250l06 2,992,726 6/1961 Simens 250106 3,204,103 8/1965 Johnson et a1. 250106 3,230,374 l/l966 Jones et a1 25084 X 3,253,152 5/1966 Lahr 250106 ARCHIE R. BORCHELT, Primary Examiner. 

1. AN ARTICLE, USEFUL AS AN AREA SOURCE OF RADIOACTIVITY, HANDLEABLE WITHOUT DANGEROUS RADIOACTIVE MATERIALS TRANSFER TO ONE''S FINGERS, SAID ARTICLE COMPRISING A MALLEABLE AND DUCTILE METAL SUBSTRATE LAYER HAVING A BRINELL HARDNESS NUMBER NO GREATER THAN 75 AT ROOM TEMPERATURE, AND A COMPACT MONOLAYER OF A MULTITUDE OF DISCRETE SPHEROIDAL ELEMENTS BETWEEN 5 AND 250 MICRONS IN DIAMETER PRESSED INTO SAID METAL SUBSTRATE LAYER TO A PARTIAL EXTENT AT LEAST EQUAL TO HALF THE DIAMETER OF SAID ELEMENTS, AT LEAST 90 WEIGHT PERCENT OF SAID ELEMENTS BEING WITHIN A LIMITED SIZE RANGE HAVING AN UPPER LIMIT OF DIAMETER NO LARGER THAN 30% GREATER THAN THE LOWER SIZE LIMIT OF DIAMETER WITHIN SAID 90 WEIGHT PERCENT PORTION OF SAID ELEMENTS, SAID SPHEROIDAL ELEMENTS EACH CONSISTING ESSENTIALLY OF A HARD AND BRITTLE SPHEROIDAL CORE CONTAINING RADIOACTIVE MATERIAL FIXED THEREIN AND A REINFORCING CONCENTRIC METAL VENEER PLATING THEREABOUT BETWEEN 0.2 AND 5 MICRONS THICK AND NO THICKER THAN ABOUT 10% OF THE DIAMETER OF THE CORE, SAID METAL OF SAID VENEER PLATING BEING LESS DUCTILE THAN THE METAL OF SAID METAL SUBSTRATE LAYER AND MORE DUCTILE THAN THE MATERIAL OF SAID BRITTLE SPHEROIDAL CORE, SAID METAL VENEER PLATING FURTHER HAVING A BRINELL HARDNESS NUMBER AT ROOM TEMPERATURE IN EXCESS OF THAT FOR THE METAL OF SAID METAL SUBSTRATE LAYER, SAID SPHEROIDAL ELEMENTS BEING EACH GRIPPED LATERALLY IN SAID LAYER STRUCTURE AND HELD IN SAID METAL SUBSTRATE LAYER BY LATERAL COMPRESSIVE FORCES EXERTED THEREUPON BY SAID METAL OF SAID METAL SUBSTRATE LAYER, THE RADIOACTIVE MATERIAL FIXED IN SAID CORES OF SAID SPHEROIDAL ELEMENTS AND THE CONCENTRATION OF SAID SPHEROIDAL ELEMENTS PER SQUARE INCH OF SAID METAL SUBSTRATE LAYER BEING SUCH THAT AT LEAST ONE MICROCURIE OF RADIOISOTOPE PER SQUARE INCH OF SURFACE OF SAID SHEET ARTICLE IS PRESENT. 